结论

1,alter table drop unused columns checkpoint适用于减少不必要的undo空间消耗

2,checkpoint选项可取值有：
    不取值，默认在512条记录后发生检查点动作
    大于表记录个数，表明在处理完所有表记录后发生检查点
    小于表记录个数，表明在指定表记录后发生检查点

3，如果checkpoint选项后产生的检查点动作被中断，仅truncate table,drop table,alter table drop unused columns continue语句可以运行

4，alter table drop unused columns continue即恢复继续执行被中断的alter table drop unused columns checkpoint继续下去    

测试

1,数据库版本
SQL> select * from v$version where rownum=1;


BANNER
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production


2，开启检查点信息到告警日志
SQL> show parameter checkpoint


NAME                                 TYPE                   VALUE
------------------------------------ ---------------------- ------------------------------
log_checkpoint_interval              integer                0
log_checkpoint_timeout               integer                1800
log_checkpoints_to_alert             boolean                FALSE


SQL> alter system set log_checkpoints_to_alert=true;


System altered.


[oracle@seconary trace]$ tail -f alert_guowang.log 
  Current log# 2 seq# 356 mem# 0: /oracle/oradata/guowang/redo02.log
Fri Oct 16 08:18:16 2015
Thread 1 advanced to log sequence 357 (LGWR switch)
  Current log# 3 seq# 357 mem# 0: /oracle/oradata/guowang/redo03.log
Fri Oct 16 08:27:17 2015
ALTER SYSTEM SET log_checkpoints_to_alert=TRUE SCOPE=BOTH;
Fri Oct 16 08:28:44 2015
Beginning log switch checkpoint up to RBA [0x166.2.10], SCN: 11696352
Thread 1 advanced to log sequence 358 (LGWR switch)
  Current log# 1 seq# 358 mem# 0: /oracle/oradata/guowang/redo01.log


SQL> alter system checkpoint;


System altered.


Fri Oct 16 08:29:15 2015
Beginning global checkpoint up to RBA [0x166.1044.10], SCN: 11698731
Completed checkpoint up to RBA [0x166.1044.10], SCN: 11698731
Completed checkpoint up to RBA [0x166.2.10], SCN: 11696352


3,创建测试表并插入数据
SQL> conn scott/system
Connected.
SQL> create table t_checkpoint(a int,b int);


Table created.


SQL> insert into t_checkpoint select level,level from dual connect by level<=100000;


100000 rows created.


SQL> commit;


Commit complete.


4，指定测试表B列为不可用
SQL> alter table t_checkpoint set unused column b;


Table altered.


5，可见指定checkpoint选项后会发生一个检查点动作


SQL> alter table t_checkpoint drop unused columns checkpoint 500;


Table altered.


Fri Oct 16 08:34:11 2015
Beginning log switch checkpoint up to RBA [0x167.2.10], SCN: 11720087
Thread 1 advanced to log sequence 359 (LGWR switch)
  Current log# 2 seq# 359 mem# 0: /oracle/oradata/guowang/redo02.log




6,如果正在进行检查点动作，被中断，会如何呢？
SQL> drop table t_checkpoint purge;


Table dropped.


SQL> create table t_checkpoint(a int,b int);


Table created.


SQL> insert into t_checkpoint select level,level from dual connect by level<=100000;


100000 rows created.


SQL> commit;


Commit complete.  


SQL> alter table t_checkpoint drop unused columns checkpoint;


alter table t_checkpoint drop unused columns checkpoint
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-01013: user requested cancel of current operation






SQL> SQL> 


SQL> select table_name,status from user_tables where table_name='T_CHECKPOINT';


TABLE_NAME                                                   STATUS
------------------------------------------------------------ ----------------
T_CHECKPOINT                                                 VALID


可见如果检查点进程被中断，不能进行alter table add操作，必须先要让检查点运行完成，即运行
  alter table drop columns continue
SQL> alter table t_checkpoint add c int;
alter table t_checkpoint add c int
            *
ERROR at line 1:
ORA-12986: columns in partially dropped state. Submit ALTER TABLE DROP COLUMNS
CONTINUE




SQL> 


SQL> host oerr ora 12986
12986, 00000, "columns in partially dropped state. Submit ALTER TABLE DROP COLUMNS CONTINUE"
// *Cause:  An attempt was made to access a table with columns in partially 
//          dropped state (i.e., drop column operation was interrupted).
// *Action: Submit ALTER TABLE DROP COLUMNS CONTINUE to complete the drop
//          column operation before accessing the table.


select查询也不成
SQL> select count(*) from t_checkpoint;
select count(*) from t_checkpoint
                     *
ERROR at line 1:
ORA-12986: columns in partially dropped state. Submit ALTER TABLE DROP COLUMNS
CONTINUE


truncate操作是可以成功的
SQL> truncate table t_checkpoint;


Table truncated.




alter table drop columns continue用于继续执行被中断的检查点选项的删除列操作
SQL> alter table t_checkpoint drop columns continue;


Table altered.


SQL> alter table t_checkpoint add c int;


Table altered.

出自：

oracle 11g之alter table drop unused columns checkpoint删除表不可用列系列二_ITPUB博客